Mechanisms of sunscreen failure

Sunscreen is used as a primary strategy to prevent sunburn and later skin c ancer. However, sunscreen use has paradoxically been associated with the in creasing incidence of skin cancer. One explanation for this puzzling observ ation is sunscreen failure (sunburn in the setting of sunscreen). Our purpo se was to evaluate mechanisms of sunscreen failure in a sunscreen-using pop ulation. We carried out an epidemiologic comparison of sunburned and nonsun burned beachgoers who used sunscreen. We found that men were less likely to use sunscreen than women (chi (2) = 11.3, df = 1, P = .001), and when it w as used, men were less likely to apply sunscreen to all sunlight-exposed sk in (chi (2) = 18.4, df = 1, P = .0001). Swimmers who used sunscreen were si gnificantly more likely to be sunburned compared with nonswimming sunscreen users (Fisher exact test, df = 1). Sunscreen may fail to prevent sunburn i f it is washed off during swimming or if it is not applied to all exposed s kin. Epidemiologic studies that link sunscreen use to skin cancer should ev aluate whether sunburn occurred in this setting.